I've been pleased with my Nikons. I'm looking forward to the new Nikons. In fact, I intend to try the same scope. I wish they offered a mil dot and 1/4" clicks in thier higher power scopes. Don't have any personal experience with the others.
I decided I had to have one with more x a while ago. I called Bushnell about thier 6x24. Seems there are two models. One with only 20 something in adjustment. The other with 40 something. Unfortunately, nobody I talked to (including a couple of thier customer service people) could agree which was which. Plus, it's a realllly loooong scope. And, I think it had 1/8" clicks.
Dont know much about Sightron. They seemed to be priced higher than other brands I'm more familiar with.
I finally bought a Leupy 6.5-20. I would have went with a Nikon if they'd been available and had 1/4" clicks.
I have a 4-12 Monarch AO I intend to replace with the New 4-16 Mil dot. I also have a 4.5-14 Buckmaster. They're both good. But the Monarch is better. Further, it's a scope my experience has made me confident in. Confidence counts!
For me it came down to 1/4" clicks at higher power. If the Nikon made 'em I'd have bought one.
Far as the power range goes. I like to shoot milk jugs. The twenty x really helps when you get out to lk (on small targes). However, I frequently turn it down to 16 to help spot hits. I've found 12x just a bit shy for small targets at 1k or so (it's plenty for game). 16 is good, 20 helps. As far as targets, my Leupy has fine cross hairs. This helps a lot. For game, the regular reticle and mil dot are great.
Rambled a bit... Hope there's something of value in there. Bottom line, of the three, I'd buy the Nkon.